There are two types of multimeters.
How to test voltage on a solar panel with multimeter.
You want to choose a voltage range capable of displaying the maximum possible voltage of the panel in open circuit this means that if you have a panel rated for 20 volts you should set the multi meter.
Switched multimeter this type of multimeter manually switches between the ranges to get the most accurate reading.
To test a 18v solar panel voltage output directly put your solar panel in direct sunlight set your multi meter to the dc volts setting.
Some exceptions may apply based on panel type.
A multimeter is a device that you can use to test the voltage and current of any device.
Including the solar panels.
The next step is to ensure that you select a voltage range that can display the possible maximum voltage of your solar panel in an open circuit.